Process of preparing indurated pellets of iron ore fines



United States Pa n we PROCESS OF PREPARING INDURATED PELLETS OF IRON ORE FINES Fred D; De'Vaney, Hibbing, Minm, assigner to Erie Min- Company, Hibbing, MimL,a corporation of lAiHDE- sota I a No Drawing. Ap li ation January 28,1952,

Serial No. 268,681 I p rollingoperation and thereafter heat-treating (indurating)' the pellets .in a shaft-type furnace. to impartto' them a considerable mechanical strength or ruggedness An article by C. V. Firth published in the April 20y 1944,- issue. a

of -Proceedings of the Blast Furnace and Raw Materials Committee, A. I. M. M. 13., provides a general description ofthe'pelletizing process. The present invention-is more particularly concerned with improvements inthat typeof pelletizing process wherein the small balls or pellets contain uniformly dispersed therethrough a normally solid exothermically oXidiZable component in an amount insuificient per se to yield by its oxidation all of the heat required for carrying .out. the induration step,

The invention comprises process and product aspects.

Because of the peculiar heating cycle inherent in induratingsuch pellets in a shaft furnace, the pellets have a great tendency to disintegrate during the-earlier stages v of the; induration procedure which tendency; is. not encountered in more gradual heating-as in. a tunnel kiln or.

the like. Thus, they may exfoliate orthey may. explosively rupture during the rapidheatingr- Moreover, such:

pellets are readily susceptible, while in moist state, to deformation and to formation of aggregations of pellets, as when, prior to incipient induration, the moist pellets are subjected to the pressure of a considerable mass of superimposed pellets.

It has now been found that the' above disadvantages can be. avoided by homogenously incorporating with the moist finely divided ore particles, prior to balling up, a very small amount of bentonite. The amount of bentonite so used must be controlled within narrow limits,- viz.-, within the limits 0.1% and 1.0% byweight based onthe dry weight of the ore particles. Less than 0.1% bentonite does not produce a useful result. If more than 1.0% bentonite is used the resulting moist mixture when sub jected to balling-up in the balling drum quickly balls up' but the resulting product consists. of undesirably small ballswhich cannot be made to grow, by accretion, to a size. useful in feeding the blast furnace. The optimum amount .of bentonite addition is about 0.5% byweight, i. e., about 10.0 pounds per long ton, dry weight, of the orefines to be pelletized.

Ore concentrates pellets containing 0.l%1.0% of bentonite are not sticky as freshly formed, and the freshly formed pellets do not acquire stickiness during. such handling. as is required for transferring the. pellets to the top of the induration furnace and for laying down a layer of the moist pellets on the stockline of the column of pellets undergoing heat-treatment in the furnace. This small content of bentonite very materially adds to the mechanical strength or ruggedness of the raw (i. e., freshly formed and unheated) pellets, which added .mechanica'l strength carries through the drying ofthe pellets anddnto the incipient indt irationstag'e of the heat treatment'. Very surprisingly th'is' small'content'of bentonite materially increases the -fired' strength of the pellets, that is i to say, the ultimate mechanical 's'tr'ength and 'ru-'g gedness of the" discharged fullyl indu'rate'd and cooled pellets. This fired strength is not diminishedby weather ing of the pellets. This increase in fired s'tfehgth 'ilsuallyamounts to betwe'en1-0%' and 20%.

'Hentonite, as used in the present relation, is m ch sup'eriontoothe'r types of QIaythe superiorit being: due in part to the gel-forming properties of bentoniteand part to the fine particle size -of the bentonite; whiclrlat ter' has about '7 times the surface areaof the next-nearest clay;

Average number of. grains per linear centimeter}- Bentonite exh'ibits greater mobility" in themoist' state nd: tends to fitment over" a tremendous snr'face area. This iii-the case of b'erfitonitecofitaining iron are ellets, reacts, at ind'u'r'a'tion "temperature, with Faoslor resor- V or/and with gan ue comp nents to prsdncs a th'in coating of slag coverin the iron oxide rains and. firmly. bonding tliernas's together;

Meteor/er, man iron ore. concentrates are defi'ciehtin' alumina, soth'at' the small. a'dditionofhentonite contributes a little needed alumina.-

to the fired product.

It has been found that use of bentonite is attended by two. desirable results", vi'z, .(a') the samecont'ributsto the-tsmporarystrength of the pellets, i. e. to the ell'ef as freshly formed and during handling of the latter an through the dryin'gout offthepellets in the beg ning of the induration operatien'which dryin period m rks the, level of greatest fragility of the pellets, an (I i) the same contributes a remarkahleincrease in the finaher fired, strength of the pellets. This increase in fired strengthusually amounts to an improvement of between l0% and 20%. In a case where the dry strength of the pellets is of'the order of 1 lb. per pellet in the absence of hen tonite, use of bentonite Within the amount rangspecifiedf increases the dry strength to 20 lbs. or moresonietimes to as much as--50 lbs; perpellet;

' The improvement in fired strength through use of bentonite isin sharp contrastwith the street of other earthy materials such. asbarite and claysother insabentonite. Ordinary clays do not improve either the fired strength or 't hedry strength" ofanironore pellet, While use of barite depreciates rather than improves the; strength ofthepellets. v

Useiof bentonite in iron iorepellets normally increases. the drop test value from an average of 8-5 roan average of 91.5%. This. de'rease in fines production during indura'tion improvesthefurnacing operation by decreasing. the furnace pressures materially. v I

' Thein'v'ent'ionwill now be described in greater partic'u' larify with reference to the following'illust'rative exam les;

Example I A. The starting, material was a filter cake of magnetic concentrates obtained from a magnetic moi-lite; round" in the-Mesabi range. The" material analyzed as" follows:

the iron being present mostly in the form of magnetite, F6304, and the particle size distribution was as follows: essentially all minus 100 mesh and 68% finer than 325 mesh.

The filter cake was divided into a number of portions one of which was processed as a check. To another portion there was added bentonite in an amount to give a 0.5% concentration of the bentonite on the dry basis, and the bentonite-containing material was mixed until the bentonite was found to be homogeneously dispersed throughout the mass.

Thereupon, the pugged material was fed to a balling drum wherein the material was formed into pellets. The finished pellets varied somewhat in size within the limits 1.5 inches and 0.5 inch, averaging about 1.125 inches in diameter. They were found to have, in the natural wet state, a compressive strength of 6.2 pounds per pellet, and in the dry state (dried to 230 F.) a compressive strength of 21.5 pounds per pellet.

The resulting pellets were transferred to the top of a shaft-type indurating furnace generally similar to that disclosed in U. S. Patent No. 2,533,142 to Percy H. Royster and, at substantially room temperature (60 F.) were laid down on the stockline of a column of similar pellets undergoing heat-treatment. The furnace had a cross-sectional area of about 4.95 square feet at the mouth, and was fed at a rate to yield 2150 pounds per hour of indurated and cooled, dry pellets. The gases moving from the combustion chamber to and into the column at the level of the bottom of the upper of the two chambers of the furnace were maintained at a temperature of about 1850" F. by suitable adjustment of the rate of feed of the fuel. The average fuel oil feed was 2.5 gallons/hour. The rate of blowing of the air was about 452 C. F. M. (cubic feet per minute). The top temperature attained by the pellets in the upper chamber of the furnace was about 2340 F., which top temperature was attained about 12 inches below the stockline. At the level at the bottom of the upper chamber the pellets had a temperature of about 1925 F. The exit gas temperature was estimated as 245 F.

The product discharged from the induration furnace at a temperature of about 275 F., and was found to have the following screen analysis:

This product was subjected to a drop test in which 100 pounds of pellets, all 0.5 inch diameter and larger, were dropped five times through a pipe 32 feet tall onto a steel plate, and then screened at mesh and the amount of minus 10 mesh material determined by weighing. Pellets to be suitable for lake shipment must not show more than 12% by weight of minus 10 mesh material when tested by this method. The above-described fired product tested 91.5%, i. e., it contained only 8.5% of minus 10 mesh material.

B. The above mentioned chec portion was similarly balled up in the balling drum, fed to and indurated in the induration furnace and thereafter examined. The moist raw pellets ranged in size as above, and averaged as above in diameter, and had a compressive strength of 2.15 lbs. per pellet in the wet state and of 1.05 lbs. per pellet in the dried state. The pellets were subjected to the same induration treatment as that described above. The product discharged from the furnace had the following screen analysis:

Per cent Plus 1 inch 40.9 Plus .75 inch 27.5 Plus .5 inch 12.5 Plus 10 mesh 8.5 Plus mesh 6.2 Minus 100 mesh 4.4

and showed a drop test of about 85.4%, i. e., 14.6% of minus 10 mesh material.

It will be appreciated that use of the small amount of bentonite (Experiment A) made the wet pellets measurably stronger, and conferred a remarkably higher dry strength, that the disintegration losses during induration were smaller, and that the final product Was very materially stronger. Because of lack of degradation the furnace pressures were reduced about 60%.

In another, companion, test 22.5 lbs. bentonite were added. The resulting balls were small, not averaging more than 0.5 inch in size. Because of this size limitation the material was not further processed.

Example 11 cracks or disintegration, and withstood dropping from a height of 10 feet without breakage.

In a repetition of this experiment 1.0% bentonite was used with equally good results.

Example III The starting material was a finely divided flotation concentrate made from a tailing, containing 58.1% iron: the iron minerals were hematite and limonite with hematite predominating. The grain size was approximately all minus 100 mesh, but only 51% was finer than 325 mesh and with only a minor amount of colloidal fines.

This starting material was thoroughly mixed with approximately 11% of water and 11 pounds per long ton of anthracite fines, and divided into portions.

One portion was subjected to the balling-up operation described in Example I above. Because of the granular nature of the solids, the balls were very fragile and the amount of degradation was excessive: consequently, the test was discontinued.

Into another portion of the above mixture there was homogeneously dispersed 10 pounds per long ton of bentonite, and the resulting mixture was formed into pellets in the manner described in Example I above. The pellets averaged M1 inch in diameter. A test showed that in their dry state the pellets had a compressive strength of 3.5 lbs/pellet.

The pellets were transferred to the induration furnace wherein they were subjected to induration treatment after the manner described in Example I above. Fuel oil in the average amount of 2.8 gallons/hour was fed to the combustion chamber to maintain the temperature of 1950 F. in the air-gaseous combustion products mixture exiting from the combustion chamber and into the bottom of the upper chamber of the furnace.

There was little degradation in the furnace, and the fired product contained less than 5% of minus 10 mesh material. The drop test on this material showed about 92% on 10 mesh.

The use of an exothermically oxidizable component in the bentonite-containing pellets makes possible the occurrence, in the upper part of that portion of the column occupying the upper chamber of the furnace, a top temperature materially higher than that of the air-gaseous combustion products mixture introduced (from the combustion chamber) at the bottom of the upper chamber. This flash top temperature preferably is caused to be sufiiciently high that sintering or partial fusion occurs in the interiors of the pellets, while the exteriors of the pellets are swept by gases which while being hot have a cooling effect on said exteriors preventing the latter from becoming molten or sticky. Examination of pellets so indurated shows that the particles resident in the exterior shells of the pellets are not materially sintered together whereas the particles occupying the interiors of the pellets have experienced partial fusion. Fused slag is readily discernible in the pellets interiors, said slag being in the interstices between, and bonding together, the iron oxide particles. This slag bond between iron oxide particles is very strong. The bentonite tends to produce a slag having a melting point materially lower than if the normal ore components only are present. Hence, the slag bond develops at a temperature below dangerous melting temperature of the iron oxide.

I claim:

Process of preparing indurated pellets of finely divided iron ore concentrates which comprises forming a homogeneous plastic mixture consisting essentially of the finely divided iron ore concentrates, approximately 11% of water and from about 0.5% to not materially in excess of 1.0% by weight of bentonite based on the dry weight of the finely divided iron ore concentrates, forming the resulting homogeneous mixture into small masses, protractedly rolling the small masses in an inclined rotary drum into raw pellets averaging between 1.5 inches and 0.5 inch in a diameter, and subjecting an assemblage of the raw pellets to thermal hardening treatment during which the raw pellets are dried and thereupon are heated, in contract with an oxidizing gaseous heat exchange medium at an elevated temperature, to an induration temperature of at least 1000" C. but below the normal fusing point of the iron ore, at which induration temperature a bentonite slag bond is formed within the interiors of the petlets, thereby producing an indurated product consisting essentially of discrete indurated pellets of iron ore.

References Cited in the file of this patent UNITED STATES PATENTS 2,279,033 Dolbear Apr. 7, 1942 2,357,198 Hooey Aug. 29, 1944 2,533,142 Royster Dec. 5, 1950 2,582,386 Komarek et a1. Ian. 15, 1952 OTHER REFERENCES U. S. Bureau of Mines Report of Investigation 4829, published December 1951, by Bureau of Mines, Washington, D. 0., page 1, Figures 4 and 8 and pages 7 and 8 pertinent. Entire report 17 pages and 5 sheets of drawmg. 

